1. Field of the Invention
This invention relates to a cathodoluminescent screen for cathode-ray tubes and particularly for high-luminance tubes, such as, for example, the so-called "projection" type tubes.
2. Discussion of the Background
In a cathode-ray tube, the cathodoluminescent screen generally comprises a glass envelope used as a substrate, on which is produced at least one luminescent layer which most often consists of luminophor grains. The cathode-ray tube contains an electron source which makes it possible to produce a beam, which is accelerated and focused before bombarding the luminophor layer. Under the effect of this bombardment, the luminophors emit light, and a light image can be formed on the surface of the screen by deflecting the beam.
The resolution of the image depends in particular on focusing the beam, but it also depends on the characteristics of the cathodoluminescent screen, this screen also having effects on the light efficiency and the luminance in general.
FIG. 1 partially and diagrammatically shows, by a view in section a standard cathodoluminescent screen for cathode-ray tubes. This screen 1 comprises a glass envelope 2 forming a substrate. Substrate 2 carries a luminescent layer 3 formed for example by multiple luminophor grains L1, L2, . . . , Ln. On luminophor layer 3 is deposited in a standard way, opposite substrate 2, i.e., inside the tube, a layer 4 of an electrically conductive material, of aluminum for example, forming a film which makes it possible, on the one hand, to apply the accelerating potential as well as to drain off the charges, and, on the other hand, to reflect to substrate 2, i.e. to use, the light produced in luminophor layer 3 or the luminescent layer.
In a cathode-ray tube, glass substrate 2 generally has a thickness E on the order of 6 to 7 millimeters, and its refraction index n0 is on the order of 1.5. Under these conditions, the light emitted under the impact of an electron beam (symbolized by an arrow 13) by luminophor layer 3, by a grain L1 for example which is in contact with an inside face 5 of substrate 2, can go out through a face 6 of the latter toward the outside of the tube, only for its part whose angle of incidence (in substrate 2) is less than critical angles .phi.0, .phi.0' formed between rays R1, R1' (which represent the limiting refraction) and an axis x perpendicular to the plane of outside face 6 of substrate 2. Thus, for the light emitted from grain L1, which is propagated in the direction of outside face 6 to use and which is not included in critical angles .phi.0, .phi.0', this light undergoes a total reflection (as illustrated by ray R1) by which it is reflected to inside face 5 of substrate 2, where it is again reflected to opposite face 6, except if it encounters a luminophor grain in contact with this inside face 5; in the latter case, this light can be rediffused to use as symbolized by arrows RD1, RD2, RD3. This phenomenon, which can be repeated several times, is at the root of the creation of a halo of large dimension which tends to degrade in a significant way the contrast of images, and in another way, the light energy of the central peak, i.e., the light energy emitted along the axis perpendicular to the plane of substrate 2.
A large proportion of the light emitted by luminophor layer 3 goes outside of the tube, i.e., of substrate 2, with angles of incidence such that it is lost for use; this particularly in the application to the projection, where the rays of light going out from substrate 2 are not picked up in a large proportion by the optical means of the projection system.
FIG. 2 illustrates this situation and shows for this purpose the front of a standard cathode-ray tube T comprising a cathodoluminescent screen, such as, for example, screen 1 of FIG. 1, and diagrammatically shows lens 7 of the optical system also with a standard projection device. Under the impact at a point A of electron beam 13, a light is produced of which a part is emitted with an angle of incidence which is equal to or greater than critical angle .phi.0, as illustrated by limiting ray R1. This light can undergo multiple reflections or be rediffused to use along rays RD1, RD2, RD3, so that this light which is represented by limiting ray R1 produces the halo.
In the example shown in FIG. 2, the use consists of lens 7 which represents the optical means of a projection system. Lens 7 has an opening 8 centered on an axis 9 of tube T, axis 9 being perpendicular to the plane of screen 1.
The light emitted with an angle of incidence which is less than critical angle .phi.0 goes out of tube T, i.e., of substrate 2. Only that portion of this light is picked up for use which passes into opening 8 of lens 7, as illustrated by a useful ray RU which is emitted from point A. The other part of this light is symbolized by a ray RP going out from tube T but which does not pass through opening 8 and which is therefore lost for use, which degrades the light efficiency.
It should further be noted that the rays which are rediffused for further use and picked up by the latter can have a harmful effect, such as, for example, rediffused ray RD2 which, although parallel to axis 9, is rediffused from a point different from point A and tends to destroy the contrast.